Semiconductor wafers, sometimes called silicon wafers, are commonly used as a base on which multilevel integrated circuits are fabricated. A series of deposition and etch steps are required to form such a multilevel pattern on a semiconductor wafer. The deposition and etch steps are typically performed utilizing photolithographic optics-based processes. These processes require accurate focusing in order to produce a precise image on the semiconductor wafer. To achieve accurate focusing, wafers for the semiconductor industry must possess a high degree of surface perfection. Indeed, a non-planar semiconductor wafer surface can result in lower yield and decreased performance of the semiconductor devices, due to inaccurate focusing adversely affecting the photolithographic processes. Accordingly, surface planarity of the wafer is a critical issue in the semiconductor industry.
Chemical-mechanical polishing (CMP) is commonly used for polishing or “planarizing” the front face of a semiconductor wafer. A CMP process may be used to remove rough spots and irregularities from the wafer and to produce a planar surface and substantially uniform thickness of the wafer.
The process steps (using lithography, deposition, and etching) performed to create each layer of the multilevel integrated circuit can result in a multiplicity of irregularities on the wafer surface. It is crucial that these irregularities be removed so that processing can continue to develop new layers or circuitry without loss of focus in the lithography and so that accurate interconnections can be formed between the layers. Accordingly, in addition to polishing the wafer surface, CMP may be employed to planarize the topography of the interconnect layers of the electrical circuitry above the wafer surface.
FIG. 1 shows a diagram of an exemplary prior art CMP apparatus 20 for polishing a semiconductor wafer 22. In general, exemplary CMP apparatus 20 includes a platen 24 upon which a polishing pad 26 is mounted. Semiconductor wafer 22 may be mounted in a wafer carrier (not shown) with the integrated circuit side facing toward polishing pad 26. To planarize wafer 22, polishing pad 26 is brought into contact with semiconductor wafer 22. While wafer 22 presses against polishing pad 26, at least one of platen 24 and the wafer carrier moves relative to the other to move wafer 22 across polishing pad 26. As wafer 22 moves across polishing pad 26, material is continuously removed from wafer 22. Polishing pad 26 may be pre-soaked and may be continually re-wet with a slurry that has a variety of abrasive particles suspended in a solution.
One problem with CMP processing is that the slurry and abraded materials tend to glaze the surface of polishing pad 26. This accumulation of debris reduces the surface roughness and adversely affects polishing rate and uniformity. To counter this problem, CMP apparatus 20 includes a conditioning apparatus 28 for “conditioning” polishing pad 26. In particular, conditioning apparatus 28 is utilized to roughen the surface of the polishing pad 26.
Conditioning apparatus 28 includes a conditioning arm 30 with an end effector 32 that includes an abrasive disk (not visible). A drive mechanism 34 is coupled at an opposite end of conditioning arm 30 from end effector 32. Drive mechanism 34 is in communication with end effector 32 to cause rotational movement 36 of end effector 32. In addition, conditioning apparatus 28 includes means for vertical movement 38 of end effector 32 to bring end effector 32 into contact with polishing pad 26. Conditioning arm 30 may also include means for rotational movement 40 about axis B, so that end effector 32 is allowed to sweep in a radial direction across a predetermined portion of polishing pad 26. System control may be via a controller 42 which receives conditioning parameters to control rotational movement 36, vertical movement 38, and rotational movement 40 about axis B.
Recent improvements in conditioning apparatuses entail the inclusion of a gimbal mechanism interfaced with end effector 32 so that the abrasive disk of end effector 32 remains level, thus parallel to polishing pad 26. The object of such a mechanism is to yield more uniform, thus efficacious, conditioning of polishing pad 26.
There are a number of problems associated with conventional conditioning apparatuses, such as conditioning apparatus 28. For example, the conditioning pressure of end effector 32 against polishing pad 26 may differ from that which is desired leading to undesirably slow and inefficient conditioning, or conversely, leading to excessive or non-uniform, conditioning that ultimately damages polishing pad 26.
The introduction of vertical movement 38 in some prior art devices can yield undesirably high internal friction. Accordingly, the force developed by the mechanism producing vertical movement 38 and applied to the surface area of end effector 32 may be “absorbed” by the high internal friction of the mechanism, and is thus not transmitted to end effector 32. This friction can contribute to significant error, in terms of an effective reduction in the actual conditioning pressure applied by end effector against polishing pad 26. Consequently, high friction contributes to imprecision in controlling the amount of conditioning pressure applied by end effector 32 against polishing pad 26. If the conditioning pressure is too low, the rate of conditioning of polishing pad 26 may be undesirably slow and inefficient.
In addition, the conditioning pressure of end effector 32 against polishing pad 26 is not currently measured in prior art systems. Thus, it is difficult to compensate or account for the absorption of force due to internal friction. Even if the absorption of force by friction is somehow compensated for, the conditioning pressure can change over time due to mechanical wear of internal components within end effector 32. Mechanical wear can cause a change to internal friction in the mechanism producing vertical movement 38, further introducing an error component to the conditioning pressure.
Typical conditioning apparatuses do not include position feedback of end effector 32 relative to the polishing pad 26. Accordingly, if there is a mechanical failure of components within end effector 32 that causes the distance between end effector 32 and polishing pad 26 to differ from what is expected, this difference may go undetected. Such a situation is undesirable because the polishing pad 26 may become damaged during the conditioning process, the rate of conditioning may be undesirably slow, or conditioning may not occur at all. Unfortunately, such a scenario cannot be detected without interfering with the conditioning process and/or incurring excessive and costly down time.
Yet another problem with prior conditioning apparatuses is that the force range of the mechanism producing vertical movement 38 is limited by the amount of pressure that can be exerted on the internal mechanisms of end effector 32. An undesirable limitation of the force further leads to undesirably slow and inefficient conditioning.